fixes circularity check in dependencies

The check for circular loading should depend on a stack of files being
loaded at the moment, rather than the collection of loaded files.

This showed up indirectly in #16468, where a misspelled helper would
incorrectly result in a circularity error message.

References #16468

activesupport/lib/active_support/dependencies.rb

@@ -30,6 +30,10 @@ module Dependencies #:nodoc:
     mattr_accessor :loaded
     self.loaded = Set.new
 
+    # Stack of files being loaded.
+    mattr_accessor :loading
+    self.loading = []
+
     # Should we load files or require them?
     mattr_accessor :mechanism
     self.mechanism = ENV['NO_RELOAD'] ? :require : :load
@@ -317,6 +321,7 @@ def depend_on(file_name, message = "No such file to load -- %s.rb")
     def clear
       log_call
       loaded.clear
+      loading.clear
       remove_unloadable_constants!
     end
 
@@ -329,6 +334,7 @@ def require_or_load(file_name, const_path = nil)
       # Record that we've seen this file *before* loading it to avoid an
       # infinite loop with mutual dependencies.
       loaded << expanded
+      loading << expanded
 
       begin
         if load?
@@ -351,6 +357,8 @@ def require_or_load(file_name, const_path = nil)
       rescue Exception
         loaded.delete expanded
         raise
+      ensure
+        loading.pop
       end
 
       # Record history *after* loading so first load gets warnings.
@@ -475,7 +483,7 @@ def load_missing_constant(from_mod, const_name)
         expanded = File.expand_path(file_path)
         expanded.sub!(/\.rb\z/, '')
 
-        if loaded.include?(expanded)
+        if loading.include?(expanded)
           raise "Circular dependency detected while autoloading constant #{qualified_name}"
         else
           require_or_load(expanded, qualified_name)

activesupport/test/autoloading_fixtures/typo.rb

@@ -0,0 +1,2 @@
+TypO = 1
+

activesupport/test/dependencies_test.rb

@@ -157,6 +157,31 @@ def test_circular_autoloading_detection
     end
   end
 
+  def test_ensures_the_expected_constant_is_defined
+    with_autoloading_fixtures do
+      e = assert_raise(LoadError) { Typo }
+      assert_match %r{Unable to autoload constant Typo, expected .*activesupport/test/autoloading_fixtures/typo.rb to define it}, e.message
+    end
+  end
+
+  def test_require_dependency_does_not_assume_any_particular_constant_is_defined
+    with_autoloading_fixtures do
+      require_dependency 'typo'
+      assert_equal 1, TypO
+    end
+  end
+
+  # Regression, see https://github.com/rails/rails/issues/16468.
+  def test_require_dependency_interaction_with_autoloading
+    with_autoloading_fixtures do
+      require_dependency 'typo'
+      assert_equal 1, TypO
+
+      e = assert_raise(LoadError) { Typo }
+      assert_match %r{Unable to autoload constant Typo, expected .*activesupport/test/autoloading_fixtures/typo.rb to define it}, e.message
+    end
+  end
+
   def test_module_loading
     with_autoloading_fixtures do
       assert_kind_of Module, A

activesupport/test/dependencies_test_helpers.rb

@@ -13,6 +13,7 @@ def with_loading(*from)
     ActiveSupport::Dependencies.autoload_paths = prior_autoload_paths
     ActiveSupport::Dependencies.mechanism = old_mechanism
     ActiveSupport::Dependencies.explicitly_unloadable_constants = []
+    ActiveSupport::Dependencies.clear
   end
 
   def with_autoloading_fixtures(&block)